Aedes triseriatus, the treehole mosquito, is the primary vector of La Crosse virus (LAC), the most common form of human encephalitis in the midwest. Control and monitoring of this mosquito are exceptionally difficult. We propose continuation of genetic and ecological research on vector competence, in the expectation that an "Achilles' heel" may be discovered and exploited. (1) Studies on genetic variability involve establishment of a formal genetics (morphological mutants, allozymes, linkage maps), analysis of gene frequency differences in geographic populations, and clarification of relation to sibling species such as Adedes hendersoni (a non-vector of LAC). (2) Ability to transmit LAC differs greatly in different populations (10-90%); genetics of this trait will be determined and genetically marked virus-refractory strains will be developed. Preliminary evidence indicates that LAC is deleterious to the mosquito, resulting in field selection of resistant forms; pathology (EM, FA) and fecundity studies will be used to test this hypothesis. (3) A gregarine protozoan pathogen, Linkesteria barretti, is ubiquitous in field populations of A. triseriatus, with infection rates from 1-90%. Effect of the gut parasite on ability to transmit LAC and Dirofilaria will be determined. There is promising evidence that geographic strains or other species of Lankesteria could be used for biological control; laboratory and field tests will be made. (4) Egg diapause is controlled by photoperiod; marked differences occur in photoperiod sensitivity in different geographical strains. The genetic basis of this trait will be determined. Genetic control by release of non-diapausing strains will be attempted. (5) Dynamics of field populations in experimental woodlots in northern Indiana will be studied in a long-term program. For adult mosquitoes, the focus is on population size and longevity, vertical distribution, female host choice and male dispersal. Larval studies are on treehole productivity (stem flow, leaf litter). Life tables and population models will result. Our WHO Reference Center, with 35 species and 250 strains of Aedes, will continue.